1. Field of the Invention
The present invention relates to a sliding material having a high strength and a method for producing the same. More particularly, the present invention provides a boronized sliding material having excellent sliding characteristics due to boronizing and also having high strength. In addition, the present invention provides a method for strengthening a boronized material.
2. Description of Related Art
Since boronizing enables extremely hard borides to form, it can be applied to the hardening treatment of various sliding materials. They are, for example, ferrous materials which are subjected to a bending load and compression load by a vane, or the like, of an oil pump or cooler, and, which are brought into contact with an aluminum-alloy. A sliding shaft, a bearing of an engine, and transmission parts are also examples of the above described sliding materials.
Japanese Unexamined Patent Publication No. 63-159685 filed by Taiho Kogyo Co., Ltd, proposes as a vane for a compressor, a ferrous substrate which is boronized to form a boronized layer having a hardness of from Hv 1200 to 1850. Medium carbon-steel for constructional use (S45C and S55C under the JIS designation), bearing steels (SUJ), alloyed tool-steels (SKS), and alloyed die-steels for hot-forming (SKD) are mentioned in the description as the ferrous materials. S45C is described in the example of the above Japanese publication.
In boronizing, a substrate is heated to a temperature of from 750.degree. to 950.degree. C. in a powder of boron carbide or like, followed by slow cooling. This slow cooling is conventionally used so as to avoid the disadvantages brought about by rapid cooling. If the substrate is rapidly cooled after the boronizing, then thermal strain or transformation strain will become so great that not only dimension accuracy of the substrate becomes impaired but also strain of the substrate and strain of the boronized layer are combined to cause cracking of the boronized layer.
Boronized medium carbon-steels, such as S45C, exhibit an annealed structure which consists of ferrite and pearlite. A substrate consisting of the medium carbon-steel is therefore lacking in strength because the optimum strength is obtained by quenching and tempering to form the tempered structure.
The present inventors considered subjecting the boronized and then slow-cooled S45C to quenching and tempering which is a standard heat-treatment. The strength of the substrate can be enhanced, but the dimensions of the substrate are changed by the heat-treatment. This in turn causes a problem in that the extremely hard boronized layer must be machined to restore the proper dimensions. In addition, unless extremely careful heat-treatment is carried out in the quenching, the strain on the boronized layer and the quenching strain are combined to result in quenching crack and surface crack. It is therefore difficult to apply the ordinary quenching and tempering treatment to a boronized sliding material.
If the boronized sliding material has poor strength, the design of the sliding parts is limited causing production of light-weight parts to become difficult, and, the scope of further application of boronized material to machines and parts is thus limited.